Cyanate ester resins have utility in a variety of composite, adhesive, and coating applications, such as circuit board laminates, conductive adhesives, structural adhesives, protective coatings, aerospace structures, filled molded parts, structural composites, and semiconductor encapsulants, where adhesion between the cyanate ester resin and a surface is of critical importance.
Adhesion of polymers to substrates has long been a problem in adhesive and coating chemistry and in making of polymer composites. One solution has been the use of silane coupling agents as described, for example, by Plueddemann in the book "Silane Coupling Agents," published in 1982 by Plenum (New York), pp. 1-28. Typically, silane coupling agents have the structure X--Y--SiZ.sub.3 where X is a functional group capable of interacting, or preferably, reacting, with the polymeric resin, Y is an organic linkage, and at least one Z is a reactive or hydrolyzable group capable of reacting with hydroxyl groups on the surface of the substrate. The X group bonds with the polymer network and the SiZ.sub.3 group bonds to the substrate. This provides a chemical link (covalent bonds) from the polymer to the substrate through the organic group Y and thereby improves the adhesion of the polymer to the substrate. Numerous silane coupling agents have been developed for a variety of polymeric resins, but it is believed a silane coupling agent has never been developed specifically for cyanate ester resins.
Existing silane coupling agents have been used with cyanate ester resins. For example, U.S. Pat. Nos. 5,143,785 and 5,330,684 describe cyanate ester based conductive adhesives which may incorporate silane coupling agents where the X group, shown above, is mercapto, epoxy, acrylyl, or amino. A preferred coupling agent may be 3-glycidoxypropyltrimethoxysilane which was used exclusively in the examples of U.S. Pat. No. 5,143,785. Mercapto, hydroxy, and amino groups are known to react with cyanate esters but may produce undesirable side reactions and thermally or hydrolytically unstable bonds. Additionally, amino groups react too rapidly with cyanate esters to be of practical value and are catalysts for cyanate ester cure which leads to reduced shelf life for the adhesive. The '684 patent requires that an epoxy resin be present in conductive adhesive composition. Numerous patents, see U.S. Pat. No. 4,797,454, for example, have reported that epoxy groups couple with cyanate ester groups by the formation of oxazoline groups. However, recent research results reported by Fyfe and coworkers (Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 32, 1994, pp. 2203-2221) show that the reaction of cyanate esters with epoxy compounds is very complex and does not produce oxazoline structures. These researchers report that the direct reaction of cyanate esters and epoxies provides oxazolidinone structures and that this is a minor reaction pathway. This type of complex and inefficient chemistry is undesirable for a coupling agent.